The level defects in a television image derive in particular from the optical pickup system, from the image analyzer and from the receiving tube. However, these defects in level may be also introduced by devices for correcting color registration (convergence) and geometry.
It is known that level defects may be corrected by means of a cross correction system based on simple analog signals (sawtooth, parabolic) at line frequency and at frame frequency. The effectiveness of a correction of this nature is actually assured only over the small portion of the image in respect of which the correction signals were determined and which commonly consists of a cross + of which the intersection is situated at the center of the image.
It is equally known that level defects may be corrected by means of a device for correction by points, in which the television image is divided into a particular number of correction points by means to a squared grid of constant pitch: for example a grid divided into seven equal areas in the vertical direction and in the horizontal direction, establishing 49 correction points, or a 14.times.13 grid establishing 182 correction points. Experience shows that these devices do not for their part permit the elimination of the level defects of the television image in a satisfactory manner.
The present invention has as its object in particular to provide an improved correction of level defects in a television image compared with the known devices.
This is primarily obtained by means of a device for correction by points, in which the height and width of the correction points are not all identical and are the result of establishing a squared grid which is not of constant pitch and which is devised for the corrections which are to be performed.